In producing hydrocarbons and the like from certain subterranean formations, it is now common to drill production wells at angles which are highly deviated from vertical. For example, where several wells are to be drilled from a single surface site, each well is deviated so that the bottom of the well will lie a substantial distance from the bottoms of the other wells when all of the wells are completed into the producing formation.
Further, many hydrocarbon-bearing formations may be produced more economically from a horizontal well bore due to the formation thickness, porosity, permeability, etc. As will be understood in the art, a "horizontal" well is a well which is normally formed by initially drilling the borehole from the surface in a generally vertical direction and then curving the borehole in a highly deviated or horizontal direction whereby the "bottom" of the borehole extends substantially horizontal through the production formation for a substantial distance. This substantially increases the surface area of the borehole which is in direct contact with the producing formation through which the hydrocarbons from the formation can flow into the borehole. The same holds true for wells which are to be used as injector wells in water floods, gas floods and reservoir repressurization, and the like.
Unfortunately, many of the hydrocarbon-bearing formations to be produced from deviated or horizontal wells are originally incompetent (i.e. formed of an unconsolidated matrix material such as loose sandstone or the like) or become incompetent when produced over periods of time. When producing such formations, large volumes of sand and/or other particulate material becomes entrained in the fluids and are produced therewith. This produced sand is highly detrimental to the production equipment such as the downhole pumps and surface equipment and routinely leads to high maintenance cost and substantial downtime.
One of the best known techniques for alleviating sand production involve "gravel packing" the borehole adjacent the production formation. Basically, gravel packing includes the steps of placing a fluid-permeable liner (screen, slotted pipe, etc.) within the borehole (cased or open) adjacent the production interval and then filling the annulus formed between the borehole wall and the liner with gravel or the like. When properly positioned in the annulus, the gravel supports the walls, prevents caving of loose material against the liner, and serves to restrain particulate material from the formation, e.g. sand, from flowing into the borehole with the produced fluids.
Several techniques are known for placing the gravel in the well. Probably the most commonly used of these involves mixing the gravel with a high viscosity fluid (e.g. crude oil, polymer-type, water-based fluids, and the like) to form a gravel-slurry and then circulating the slurry down the borehole. While the circulation may be either normal or reverse circulation, a typical technique flows the slurry down a tubing which supports the liner on the lower end thereof. As the slurry reaches the top of the liner, it exits the tubing through a perforated section or a "cross-over" sub and flows down the annulus around the liner. The carrier fluid from the slurry flows both into the formation and through the screened openings of the liner, the latter being returned to the surface through an annulus formed around the tubing above the liner. It is desirable that gravel be carried into and deposited in the production perforations formed in the casing (if the borehole is cased). The small openings in the liner, however, prevent the gravel from entering the liner. Accordingly, the gravel is separated from the fluid and is deposited in the annulus around the liner thereby forming the "gravel pack".
Gravel packing has achieved universal use in substantially vertical wells where gravity aids in properly distributing and settling the gravel around the liner. However, problems exist when gravel pack completions are attempted through long production zones or in highly deviated or horizontal wells. That is, when a gravel-slurry flows out of the tubing into the annulus at the "top" of a liner in a long production zone or in a deviated or horizontal well, the gravel in the slurry has a tendency due to gravity, to fall out and form a "dune" along the liner which may eventually becomes a plug thereby reducing the velocity of the gravel-slurry and its efficiency in filling the perforations (in a cased hole) and in forming a uniform pack around the liner. Also, in a horizontal wellbore, the gravel-slurry will have a tendency to flow unevenly on the bottom side of the borehole which may result in an uneven distribution of the gravel both in the casing perforations and around the horizontally positioned liner.